The present invention relates to tubes and the manufacture of tubes from alloyed steel, which tubes can be welded in order to form underwater pipelines for hydrocarbons, tubes for oil drilling and the construction of drilling platforms at sea which provide a particularly high degree of safety and for applications requiring good ductility at low temperatures and in particular low sensitivity to the grooving effect at low temperatures.
More particularly, the invention relates to exploitation of hydrocarbon deposits in arctic regions and to the manufacture of tubes of great thickness for use therein.
A known method for the manufacture of tubes of this type includes of choosing favourable compositions of steel alloyed with manganese and molybdenum, of forming sheets from the latter and of subjecting the sheets to controlled rolling.
Unfortunately, with the known method, the maximum thicknesses are limited to 30 mm and the method has a number of metallurgical drawbacks.
For example, in a tube produced by this method the isotropy of the tube decreases with thickness with the ratio of transverse resilience to longitudinal resilience being of the order of 0.6 to 0.7 with sheets having a thickness of 30 mm. This means that its transverse resilience is much less than the longitudinal resilience.
The applicant has discovered that if, instead of working the structure of the steel by rolling, the steel is shaped by centrifugal moulding and if the centrifuged product is subjected to controlled cooling and a suitable thermal treatment, one obtains a ferritic structure with very fine grains with a tube thickness substantially greater than 30 mm, even up to 150 mm, while one obtains mechanical characteristics which are high and identical in all directions, that is, isotropic. Furthermore, one obtains a centrifuged product which is quite suitable for welding.
This discovery is all the more surprising as moulded products hitherto were considered as being of inferior quality to rolled products and had more structural irregularities whereas it was thought that working or rolling breaks the casting texture and making it possible to obtain optimum characteristics at least in the direction of rolling.